The present invention relates to a fuel injection control system for a diesel engine and, particularly to a control for reducing a discharge of smoke.
Various types of fuel injection apparatus for a diesel engine have been previously proposed. One of such known fuel injection apparatus is, for example, a delivery type fuel injection pump which controls fuel injection timing and amount in an electronic manner (refer to SAE paper 860145, published in February 1986).
Referring to FIG. 6, there is shown a delivery type fuel injection pump as mentioned above. The fuel injection pump includes, within a housing 1, a low-pressure side feed pump 2, and a high-pressure side plunger pump 3 which are driven by a drive shaft 4. Inhaled by the feed pump 2 via a fuel inlet (not shown), a fuel is supplied to a pump chamber 5 arranged within the housing 1, then transferred to the plunger pump 3 via a suction passage 6 which opens to the pump chamber 5.
A plunger 7 of the plunger pump 3 has one end formed with suction slots, and the other end connected to a face cam which is formed with cam grooves. The suction slots corresponds in number to the number of cylinders of an engine, and the cam grooves correspond in number to the number of the engine cylinders. The plunger 7 reciprocates, in rotation with the drive shaft 4, by a predetermined cam lift over a roller 11 arranged to a roller ring 10. The fuel is inhaled into a plunger chamber 12 from the suction slots 8 due to this rotational reciprocating motion of the plunger 7, then transferred from a delivery port (not shown) for each cylinder to an injection nozzle (not shown) via a delivery valve (not shown).
A fuel return passage 13 allows fluid communication of the plunger chamber 12 with the pump chamber 5 of the low-pressure side. Installed therein is an electromagnetic valve 14 of the high speed response type which is driven, in accordance with an engine operating condition, by a signal (drive pulse) from a drive circuit. The electromagnetic valve 14, which is arranged to control a fuel injection, operates such that the fuel injection begins with the valve 14 closed during a compression stroke of the plunger 7, and the fuel injection finishes with the same opened. That is, a fuel injection start timing is controlled by a valve-closing timing, and a fuel injection period (fuel injection amount) is controlled by a valve-closing period.
It is well known that, in exhaust gas of a diesel engine, an oxygen concentration and a smoke concentration are mutually correlated. A fuel injection control system which utilizes such correlation is disclosed, for example, in JP-A 62-189343. This system electronically controls the fuel injection amount based on a signal derived from an oxygen sensor which is installed within an exhaust passage.
Further, it is known that the correlation between the oxygen and smoke concentrations in exhaust gas varies with characteristics of the fuel and, particularly with a specific gravity thereof (see FIG. 3).
The specific gravity of the fuel in general use is variable according to fuel specification and temperature, but the known system is not sensible of variations of the specific gravity of the fuel.
Thus, with a fuel having higher smoke concentration versus oxygen concentration in exhaust gas, the fuel injection amount is such that the smoke concentration exceeds a predetermined reference value thereof, resulting in an increased smoke concentration in exhaust gas. On the other hand, with a fuel having lower smoke concentration versus oxygen concentration in exhaust gas, the fuel injection amount is reduced even if the smoke concentration does not exceed the reference value thereof, resulting in an decreased output of the engine.